Wheel hub assemblies having wheel speed sensors are known. A conventional wheel hub assembly includes a fixed member and a rotatable member and a bearing that supports the rotatable member for rotation relative to the fixed member. In these assemblies, a sensor is mounted on the fixed member, and a target is mounted on the rotatable member. The target may comprise, for example, a plurality of circumferentially spaced magnetic poles, and the sensor may detect the movement of the magnetic poles past the sensor and produce an output signal each time a pole is sensed. The number of magnetic poles that are detected passing the sensor in a given time period can then be used to determine the speed of the wheel.
Signals from wheel speed sensors can be used for different purposes including for controlling a speedometer. These signals are also used for controlling antilock braking systems (ABS) and other traction and/or stability control systems which require accurate information about the speed of each vehicle wheel in order to control braking and/or power provided to each wheel. For example, during a braking operation, the ABS compares the signals from the wheel speed sensors of each vehicle wheel. If the ABS microprocessor sees the wheels are locked or turning at different speeds (skidding), it modulates the brakes in an effort to keep the all wheels rotating at the same rate. However, when a sensor produces an inaccurate signal, indicating for example, that one wheel is turning slower than the others, the ABS may release the brakes to that wheel even though braking is required. Similarly, erroneous sensor signals may prevent an ABS system from releasing braking when it is actually required. By keeping the wheels “unlocked” and then decelerating progressively, the braking distance is reduced.
Various environmental factors can damage the sensors and/or targets and lead to false readings. For example, even though the region around the sensors and targets is substantially sealed, road salt and/or excessive dirt or mud still may come between the sensor and the target and prevent the sensor from generating accurate readings. It would therefore be desirable to provide a wheel hub assembly that is capable of providing accurate information about the speed of each vehicle wheel even if a particular sensor and/or target is fouled or damaged.